Base peaking often occurs in an uncured rubber tread of a cap/base construction by an intrusion of a tread base rubber layer underlying an outer tread cap rubber layer into and within the outer tread cap rubber layer during the molding and curing of the tire tread in a tire mold. Base peaking occurs when the underlying uncured tread rubber layer flows into and within the lug portions of the uncured outer tread cap rubber layer as the tread rubber flows during the shaping and curing of a green, unvulcanized rubber tire in a tire mold at an elevated temperature and pressure. Such base peaking is well known to those having skill in such art.
Sometimes it is desired to minimize, by inhibiting or retarding, excessive base peaking in a tire tread.
Where a green, or uncured, rubber tire is inserted in a suitable tire mold under conditions of elevated temperature and pressure to shape and cure, or vulcanize, the tire, the uncured outer tread rubber layer flows into depressions in the mold cavity to create shaped tire tread lugs.
In practice, the underlying uncured rubber layer can physically flow together with the outer uncured rubber layer into the mold cavity to intrude into and within the formed tread lugs of the outer tread rubber layer. Such intrusion of the underlying tread rubber layer into and within the lugs of the outer tread rubber layer is sometimes referred to as “base peaking”. As indicated, this phenomenon is well known to those having skill in such art.
It is sometimes desired to retard such base peaking in the formative tread shaping and curing operation and to therefore minimize the resulting base peaking profile, or configuration, of the underlying tread rubber layer in the finalized cured tire tread.
In one aspect, it is to be appreciated that the viscosity (e.g. Mooney viscosity) of the uncured underlying rubber tread layer may be increased to resist its flow together with the outer tread rubber layer into the mold cavities with conventional compounding ingredients, such as, for example, by increasing its carbon black content. However, it is considered that such technique of increasing carbon black content tends to defeat the overall concept of such underlying tread rubber layer because it tends to increase the hysteresis of the rubber composition which typically promotes greater internal heat generation within the tire tread, and thereby higher running temperature, as the tire is being run and, further, may also promote poorer rolling resistance for the tire tread and thereby an associated reduced fuel economy for the associated vehicle.
Historically, inclusion of high crystallinity trans 1,4-polybutadiene and high crystallinity trans 1,4-polyisoprene polymers in such underlying tread rubber layer has been suggested to increase its uncured rubber viscosity and thereby reduce, or minimize, its flow with the uncured outer tread rubber layer into the configured tire mold cavity to reduce its base peaking profile during the shaping and molding of the tire tread. For example, see U.S. Pat. Nos. 5,174,838 and 5,284,195.
Use of such high crystallinity polymers has been observed as being useful in retarding base peaking because they are in a nature of thermoplastic resins in their uncured state at room temperature by virtue of their high crystallinity. Because they contain many double bonds in their polymer backbone, they can, however, be suitably blended and co-cured with elastomers contained in the rubber composition of the underlying rubber tread layer. Such inclusion of the high crystallinity polymers can create sufficient viscosity in the uncured tread base rubber composition to resist the aforesaid base peaking effect of the tread base rubber layer into the outer tread cap rubber layer, particularly into its tread lugs, during the tire molding operation.
However, for this invention, an entirely different approach is evaluated for resisting base peaking. Use of high crystallinity trans 1,4-polymers is not a focus of this invention.
For this invention, it is desired to evaluate a possibility of partially pre-curing the rubber composition of the underlying tread rubber layer to modify its morphology, in a sense of significantly and prematurely increasing its viscosity, by use of a technique which is believed to be novel and a dramatic change from past practice which is therefore considered to be worthy of evaluation for resisting or retarding and thereby minimizing the base peaking and to provide a tread of cap/base construction with a minimization of base peaking configuration within the tire tread.
In particular, it is desired to evaluate providing a rubber tire tread strip in a form of a co-extruded laminate comprised of two rubber layers, where the tread strip is comprised of a first uncured tire tread outer rubber layer and a second partially cured tire tread underlying rubber layer where said underlying rubber layer underlies (and is therefore next to and in contact with) said first uncured rubber layer.
For this evaluation, it is envisioned that the underlying tread rubber layer becomes partially cured during the rubber co-extrusion process by providing its rubber composition, as what might be termed as being a scorchy rubber composition, characterized by having a time for a T+5 Mooney viscosity rise at 121° C. in a range of from about 5 to about 20 minutes (a relatively short time) and at least 5 minutes less than the time for a time for a T+5 temperature rise at 121° C. for the rubber composition of the tire tread outer rubber layer.
It is envisioned that the rubber extrusion temperature, for the rubber extrusion process, may be, for example, in a range of from about 90 to about 130° C. with a residence extrusion time being sufficient to partially pre-cure the tire tread underlying rubber layer by the extrusion process (e.g. within the extruder) and with a minimal, if any, pre-curing of the rubber composition of the tire tread outer rubber layer.
The co-extruded tread rubber strip is then built as a circumferential tread into an assembly of a plurality of uncured rubber components and the assembly molded in a suitable tire mold at an elevated temperature and pressure to form a cured tire with the outer tread rubber layer having a lug and groove molded configuration and an intended minimal, if any, base peaking of the underlying tread rubber layer into and within the lugs of the outer tread rubber layer.
In the description of this invention, the terms “rubber” and “elastomer” where used, are used interchangeably, unless otherwise prescribed. The terms “rubber composition”, “compounded rubber” and “rubber compound”, where used, are used interchangeably to refer to “rubber which has been blended or mixed with various ingredients” and the term “compound” relates to a “rubber composition” unless otherwise indicated. Such terms are well known to those having skill in the rubber mixing and rubber compounding art.
In the description of this invention, the term “phr” refers to parts of a respective material per 100 parts by weight of rubber, or elastomer. The terms “cure” and “vulcanize”, where used, may be used interchangeably unless otherwise indicated.